This invention relates to improved propshafts for use in motor vehicles and improved center bearing mounts for use in single or multi-piece propshafts.
A motor vehicle generally utilizes a driveshaft or propshaft to transfer the mechanical force generated by the engine to the driving wheels of the vehicle, which in turn propel the vehicle. The propshaft connects the transmission, transfer case or power takeoff unit to the driving axle. Propshafts can either be single piece or multi-piece as described below. Space in the underbody of a vehicle is at a premium, and so the diameter of propshaft may have to be reduced to allow the packaging of other components. It is also desirable to minimize the weight of vehicle components to improve performance and fuel economy.
Propshafts can become dynamically unstable if operated at the rotational speed where the propshaft residual imbalance forces coincide with the propshaft first bending mode of vibration, also known as the propshaft natural bending resonance. This rotational speed is known as the propshaft critical speed. The low damping properties of typical materials used in the construction of propshafts result in bending stresses that quickly increase at resonance and may cause the propshaft to fracture. A properly designed propshaft is almost never allowed to operate at its critical speed. One of the factors influencing the bending resonance is the length of the propshaft, such that as the length increases, the bending resonance frequency decreases, and so does the critical speed.
Besides using larger diameter, lighter and more rigid materials for propshafts, one known technique used to overcome the problems associated with a long propshaft has been to split the shaft into multiple sections or pieces. Each shorter piece has a comparatively higher bending resonance. Overall, this gives the propshaft a higher critical speed.
While multi-piece propshafts overcome some of the deficiencies of single piece propshafts, they introduce other problems. For example, multi-piece propshafts tend to exhibit launch shudder. Launch shudder is a severe vibration that occurs when the vehicle starts from a standstill or is driven at low speed during high powertrain torque events. In the case of a two-piece propshaft, a stationary center support and an additional joint are introduced where the two pieces of the propshaft come together. The center joint allows the rear propshaft piece to operate at different angles from the front piece. Typically, the stationary center support attaches the front piece of the propshaft to the vehicle underbody. Since the engine and the transmission are supported on mounts, the position of the front piece of the propshaft is relatively static. However, the same cannot be said for the rear piece of the propshaft on vehicles with solid beam drive axles. Relative to the vehicle, the position of the rear joint moves vertically during operation of the vehicle. For example, when a vehicle""s payload is increased or decreased, the rear joint moves vertically as the vehicle body moves toward or away from the axle. This movement alters the joint angles on the rear propshaft piece.
In addition, the orientation of the drive axle pinon shaft can change when drive torque is applied. This movement increases the joint angles. The applied torque combined with increased joint angles can cause launch shudder vibration. Launch shudder severity typically increases with higher payload, heavier throttle application, trailer towing and operating the vehicle up a grade.
Launch shudder is much less likely to occur in single piece propshafts because the joint angles are smaller, the distance between the joints is greater and the distance between the propshaft supports is greater.
In addition to the above noted distinctions between single and multi-piece propshafts, other factors also influence the selection of a propshaft. Single piece propshafts are simpler to engineer, and thus are less expensive to manufacture compared to multi-piece propshafts. In addition, multi-piece propshafts typically are more than double the weight of comparable length single piece propshafts.
The inventor has recognized a solution to one or more of the above stated problems and achieved an inexpensive and easy way to engineer a propshaft capable of operating at current vehicle operating speeds while eliminating launch shudder.
The present invention includes a propshaft assembly that has a propshaft connected to a transmission and an axle and a floating center support that attaches the propshaft to a vehicle. The floating center support permits the propshaft to move vertically during operation of the vehicle. The present invention also includes a method for damping the natural bending resonance with a center support. The floating center support may be adapted to permit the propshaft to move vertically in response to vertical movement of an axle attached to the propshaft. Alternately, the floating center support may be adapted to selectively fix the vertical position of the propshaft when the propshaft is operating near its natural bending resonance.